1. Field of the Invention
The invention relates to an analogue, scanning or measurement probe used on a coordinate positioning machine such as a coordinate measuring machine or machine tool, to measure the position or contour of a surface.
Coordinate positioning machines are known, and include an arm and a table movable relative to each other with three linear degrees of freedom, together with three transducers which measure displacement of e.g. the arm relative to a reference position on the table. An analogue probe used on such a machine typically comprises a fixed structure, by which the probe is supported on the arm, and a movable supporting member suspended relative to the fixed structure by an assembly permitting movement of the supporting member relative to the fixed structure with three degrees of freedom. One known form of suspension assembly includes three serially mounted pairs of leaf springs which permit substantially linear movement (over a relatively small distance) of the supporting member in three substantially orthogonal directions. The displacement of the supporting member relative to the fixed structure (and therefore the movable arm of the machine, as well as, by implication, the table) is measured by three transducers within the probe, each of which includes a detector provided on the fixed structure for sensing displacement of a body rigidly connected with the movable supporting member.
In use, the movable supporting member carries an elongate stylus having a spherical sensing tip at its free end, and the machine is operated to cause relative movement between the table and the arm in order to bring the sensing tip of the stylus into contact with the surface whose position or contour is being measured. The instantaneous position of the spherical sensing tip (and therefore of any surface in contact therewith) is determined relative to the reference on the table by summing the outputs of the corresponding transducers in the probe and on the machine. Machines equipped in this way may be used to measure the position of a single point, or, alternatively, to trace the contour of the surface by operating the machine to cause relative movement between the table and the arm while maintaining contact between the sensing tip of the stylus and the surface.
2. Description of Related Art
In a preferred form of analogue probe, the transducers in the probe are provided by an optical scale rigidly connected to the movable supporting member, and an opto-electronic readhead provided on the fixed structure which measures displacement of the scale in a given direction. A problem exists however in particular with analogue probes which employ transducers of this type in conjunction with suspension assemblies of the type described above, i.e. suspension assemblies which include a plurality of pairs of parallel leaf springs, where the stylus connected to the movable supporting member is relatively long. The bending moment applied to the movable supporting member as a result of lateral forces at the sensing tip of the stylus increases with stylus length. This bending moment is inevitably transmitted to the leaf springs of the suspension assembly for the supporting member. Large bending moments at the movable supporting member therefore cause deformations of the leaf springs in the suspension assembly to the extent that the supporting member undergoes a small scale rotation in addition to the translational displacement as a result of forces applied to the sensing tip of the stylus (as a result of contact with a component under inspection). This rotation of the movable supporting member in turn causes a corresponding tilting of the optical scales connected thereto. Tilting of the scale beyond a certain angle and in a particular direction relative to its corresponding opto-electronic readhead results in a failure of the transducer. This phenomenon therefore places a practical limit on the length of styli which may be carried by the movable supporting member, and therefore a significant restriction on the different inspection tasks which may be performed.